1. Field of the Invention
The present invention relates to a method for reducing an input-side holding torque of a wave gear device in which the input-side holding torque for holding the wave generator that is an input-side member of the wave gear device is reduced so that the wave generator is not rotated by the torque applied from the load side. The present invention also relates to a rotary actuator in which the method is adopted.
2. Description of the Related Art
Rotary actuators provided with wave gear devices having a high positioning accuracy and a high reduction ratio are used in joint function mechanisms and the like of industrial robot hands. JP-A 2004-181610 and JP-A 2004-122339 disclose a robot hand provided with rotary actuators that have a wave gear device coupled to a motor shaft.
A wave gear device has a configuration in which an annular flexible external gear is coaxially disposed inside an annular rigid internal gear, and a wave generator is fitted therein. The wave generator is commonly elliptically contoured, and a state is formed in which the flexible external gear is elliptically flexed by the wave generator and in which the two gears mesh at the two ends of the elliptical shape in the direction of the major axis. The flexible external gear commonly has two fewer teeth than does the rigid internal gear. Therefore, when the wave generator is rotated by a motor, the meshing position of the two gears moves in the circumferential direction and a relative rotation corresponding to the tooth number difference is generated in the two gears. The rigid internal gear is usually fixed in place so as to prevent the rotation, and reduced rotations are outputted from the flexible external gear.
A cup-shaped flexible external gear and a silk hat-shaped flexible external gear are known as flexible external gears for a wave gear device with such a configuration. A cup-shaped flexible external gear has a cylindrical trunk, an annular diaphragm that extends radially inward from one end of the cylindrical trunk, a discoid or annular boss formed continuously on the internal peripheral edge of the diaphragm, and external teeth formed on the external peripheral surface of the other end of the cylindrical trunk. A silk hat-shaped flexible external gear has a cylindrical trunk, an annular diaphragm that widens radially outward from one end of the cylindrical trunk, an annular boss formed continuously on the outer peripheral edge of the diaphragm, and external teeth formed on the external peripheral surface of the other end of the cylindrical trunk.
Also known are wave gear devices that are referred to as a flat-type or a pancake-type wave gear devices. In these types of the wave gear devices, a cylindrical flexible external gear is disposed inside of two coaxially disposed rigid internal gears, and an elliptically contoured wave generator is fitted inside of the flexible external gear. One of the rigid internal gears has 2n (where n is a positive integer) more teeth than does the flexible external gear, and the other rigid internal gear has the same number of teeth as the flexible external gear. When the wave generator is rotated, a relative rotation is generated in the two rigid internal gears. By fixing one of the rigid internal gears in place so as to prevent the rotation, the reduced rotational output can be obtained from the other rigid internal gear.
FIG. 2 shows the relationship between the torque of the rotational input-side of the wave gear device (hereinafter referred to as “input-side torque”) and the torque of the reduced rotational output-side (hereinafter referred to as “output-side torque” or “loading torque”). Line A in the diagram indicates the relationship during the speed reduction in which the wave generator is rotated, and the reduced rotations are outputted from the flexible external gear. Line B indicates the relationship during the speed increase in which the flexible external gear is rotated, and the high-speed rotation is outputted from the wave generator. It is apparent from lines A and B that in comparison to the speed increase, the input-side torque is high with respect to the output-side torque during the speed reduction.
In the joint mechanism of a robot hand, the high torque is outputted while the motor rotations are reduced via the wave gear device. In an operating pattern in which the output torque is maintained, the input-side torque can be kept low even when the output-side torque is high. Thus, the input-side holding torque, which holds the motor shaft connected to the wave generator of the wave gear device so that the shaft is not rotated by the loading torque (output-side torque), is kept low. For this reason, the value of the electric current of the motor for holding the motor shaft in a prescribed rotational position is also kept low.
An object of the present invention is to provide a method for reducing the input-side holding torque of the wave gear device in which the input-side holding torque required for holding the wave generator, which is an input-side rotating member of the wave gear device, can be further reduced so that the generator is not rotated by the loading torque.